Conversion of three-phase power has found many industrial applications throughout this century. The proportion of converters connected to the supply grid has reached high levels in the industrialized world. In its simplest and most economical form, conversion of three-phase power is achieved by controlling the supply current in six steps per power supply cycle.
FIG. 1 is a schematic diagram illustrating a typical six-pulse AC to DC converter. Referring to FIG. 1, a typical converter 100 includes a three-phase alternating current (AC) power supply 101 coupled to each of three inductors or alternate current (AC) reactors (or inductors) 110-112, each connected to a pair of rectifiers 103 and 106, 104 and 107, 105 and 108, respectively. Rectifiers 103-108 are controlled or switched on by a controller or gate trigger circuit 102 to provide a DC output (DC voltage and DC current) to a DC load 109. Reactors 110-112 (also referred to as AC chokes or inductors) and DC load 109 (which can be considered a resistance in series with a DC reactor, sometimes in combination with a battery and/or capacitor) are combined to limit the rate of rise of current and smooth the DC ripple current, respectively. Other electrical components, typically used for protection from overvoltage and rate of change of voltage, protection, detection etc have been omitted from FIG. 1 for the purpose of simplicity and clarity.
A control voltage generated by controller 102 determines a delay angle alpha, α (also referred to as a firing angle), for rectifiers 103-108. The alpha delay angle is used to control the DC voltage and/or DC current magnitude. Each rectifier switches once per power cycle (or period). For a constant control voltage signal each of rectifiers 103-108 is triggered on at a constant delay angle of α degrees past its anode-cathode voltage crossover point (in this example, α is approximately 90 degrees as the load is considered mostly reactive), resulting in a corresponding phase shift between line voltage and line current in each phase, and a change in DC voltage (or DC current). Voltages 113-115 are phased to neutral voltages, each being displaced by 120 degrees from each other. Considering 113 as a reference 114 lags 113 by 120 degrees, 115 lags 113 by 240 degrees. Voltage V1 is equal to the difference between 113 and 115; voltage V2 is the difference between 114 and 113; and voltage V3 is the difference between 115 and 114.
Waveform 201 of FIG. 2 shows the three-phase supply line to line voltages V1, V2 and V3 connected to the AC terminals of the bridge. T0 of V1, V2, V3 in waveform 201 is referred to as the anode-cathode zero voltage crossover point for thyristors 103, 104 and 105, respectively. T0 of V1, V2, V3 in waveform 301 is referred to as the anode-cathode zero voltage crossover point for thyristors 106, 107 and 108, respectively. The delay time between anode-cathode voltage crossover point (e.g. T0 in Waveform 201) and rectifier firing signal (e.g. waveform 203-205), typically measured in degrees, is known as the alpha control angle of the three-phase bridge. In steady state conditions the alpha control angle is substantially the same for each of rectifiers 103-108. In waveform 201 this is approximately 90 degrees (shown). For a constant control voltage from controller 102 the delay angle, or alpha, is substantially the same for each power cycle. As can be seen, each rectifier begins conducting at the same delay angle to its corresponding supply voltage, producing a three-phase AC current in reactors 110-112 that is 90 degrees out of phase with the three-phase supply voltage.
Waveforms 201 and 301 of FIGS. 2 and 3 show the three-phase line to line supply voltage for the converter. Waveform 202 shows the current through rectifiers 103-105, and waveform 203-205 shows the gate trigger signals generated by controller 102 to rectifiers 103-105. Waveform 302 shows the current through rectifiers 106-108 (labeled) and waveforms 303-305 show the gate trigger signals generated by controller 102 to rectifiers 106-108. Gate trigger signals, for the positive and negative groups, are only required to turn the rectifier on in a three-phase bridge. In this example, the rectifiers are considered to be thyristors. Hence the turn on of the next device in the series of the same group will switch off the previous device in the series of the same group.
For example, in the positive group of thyristors 103-105, turn on of device 104 will turn off device 103. Turn on of device 105 will turn off device 104 and so on. Using waveform 201 as a reference, two power cycles of duration “period 1” are shown in FIG. 2, where the delay angle of thyristors 103, 104 and 105 is substantially the same in each cycle. The rectifiers belonging to the positive group, 103-105, are switched in the same sequence in each period. The current in each rectifier is phase shifted by 90 degrees from its corresponding phase to neutral supply voltage. As can be seen in FIG. 3, using waveform 301 as a reference, the delay angle of rectifiers 106, 107 and 108 are substantially the same in each cycle. Waveform 302 shows the rectifiers of the negative group of rectifiers 106-108 are switched in the same sequence in each period. The current in each rectifier is phase shifted by 90 degrees from its corresponding phase to neutral supply voltage.
FIG. 4 shows the AC current in each phase of the bridge. The resulting AC current waveform, observed through chokes 110, 111, and 112 is a quasi-square waveform with a conduction angle of substantially 120 degrees in the positive half cycle and substantially 120 degrees in the negative half cycle, substantially regardless of the alpha control angle. Waveform 321 shows the three-phase line to line voltage reference V1, V2 and V3. The resultant line current waveforms flowing through inductors 110-112 are shown in waveforms 322-324, respectively. The DC current and voltage across load 109 is shown in waveform 325 and 326, respectively. As can be seen the DC current contains 6 “pulses” per period, or a ripple of 300 Hz (in this example the supply is 50 Hz). The gate trigger signals to each rectifier, generated by controller 102, are shown in FIG. 5. Waveform 341 shows the three-phase line to line AC supply voltage V1, V2, and V3. Waveforms 342-347 show the gate trigger signals to each rectifier 103-108 respectively, shown over a minimum of four cycles.
Each of the AC line current waveforms (322-324) contains in practice at least 20% of fifth harmonic current (in addition to higher order harmonics) which contributes to voltage distortion of the AC supply at the point at which the converter is connected. Further, presence of harmonics in the AC supply can cause misoperation, misfiring or shutdown of sensitive equipment connected to the same AC bus, overvoltage due to harmonic resonance with passive components connected to the network, etc. Various worldwide regulations, standards, and user specifications require converter manufacturers to substantially reduce the harmonic currents generated by new equipment (i.e. IEEE 519).
There are various harmonic mitigation techniques that can be used. In one conventional approach, a harmonic filter is utilized that is tuned to the frequency of the problematic harmonic on the network. A filter connected at the terminals of the three-phase bridge provides a low impedance shunt pathway at the frequency of the harmonic targeted, but high impedance for all other frequencies, thereby preventing the harmonic travelling upstream to the AC supply network and affecting other users connected to the same supply. However, as harmonic currents increase with load, there is generally a need to switch filters in discrete steps to compensate for changes in harmonic currents, and to avoid an adverse effect on the power factor seen by the supply network due to the inserted filter. Also, expensive and inefficient blocking reactors may be required to avoid filter overloading by harmonics produced elsewhere in the plant or in the supply network. Generally a plant fitted with filters requires preliminary studies to be made and careful designs to be completed before installing new equipment. Filters can reduce harmonics if properly designed, but can also amplify harmonics when changes to the plant supply are made. Harmonic filters are also a cost burden that a client must pay in addition to the original purchase of electronic equipment in order to comply with standards.
In another conventional approach, phase shifting of the AC supply voltage is used as a method to cancel harmonics. With this method, three incoming phases are split into two groups of isolated three phases and phase shifted with respect to each other by 30 degrees. If the converters equally share the load, the addition of their AC currents may cancel the fifth harmonic current. However, phase shifting of the AC supply requires the use of one or more transformers. This substantially increases the costs, operating losses, and equipment size and weight of converting equipment. A typical three-phase transformer typically costs and weighs more than the AC/DC converter that is connected to it. Despite the significant disadvantages associated with both of these methods, they remain the dominant techniques of alleviating harmonic distortion in AC power conversion systems.